Cell walls as reservoirs of potassium ions for reversible volume changes of pulvinar motor cells during rhythmic leaf movements.

The laminar pulvinus of primary leaves of Phaseolus coccineus L. was investigated with respect to the total K(+) content, the apoplastic K(+) content, and the water potential of extensor and flexor sections in relation to the leaf positions in a circadian leaf-movement cycle, as well as the cation-exchange properties of isolated extensor- and flexor-cell walls. Turgid tissue showed a high total but low apoplastic K(+) content, shrunken tissue a low total but high apoplastic K(+) content. Thus, part of the K(+) transported into and out of the swelling or shrinking protoplasts is shuttled between the protoplasts and the surrounding walls, another part between different regions of the pulvinus. The K(+) fraction shuttled between protoplasts and walls was found to be 30-40% of the total transported K(+) fraction. Furthermore, 15-20% of the total K(+) content of the tissue is located in the apoplast when the apoplastic reservoir is filled, 5-10% when the apoplastic reservoir is depleted. The ion-exchange properties of walls of extensor and flexor cells appear identical in situ and in isolated preparations. The walls behave as cation exchangers of hhe weak-acid type with a strong dependence of the activity of fixed negative charges as well as of the K(+)-storing capacity on pH and [K(+)] of the equilibration solution. The high apoplastic K(+) contents of freshly cut tissues reflect the cation-storing capacity of the isolated walls. We suggest that K(+) ions of the Donnan free space are used for the reversible volume changes (mediating the leaf movement) mainly by an electrogenic proton pump which changes the pH and-or the [K(+)] in the water free space of the apoplast.